Differential amplifiers are used for many functions, including sensing circuits and the like. For example, differential amplifiers are commonly used for sensing a current level. In a relatively common measurement configuration, a sense resistor is placed in a current path and an amplifier circuit senses voltage across the sense resistor. According to Ohm's law (V=IR), the sensed voltage represents or is otherwise easily converted to another signal which represents the sensed current level.
The implementation of the amplifier circuit depends on the application and system design specifications. The series sense resistor usually has a relatively small resistance, such as less than 1 Ohm (Ω) (e.g., 10-100 milliohms (mΩ) or the like), to avoid significant impact on the current being measured. Thus, the amplifier circuit is usually configured with a relatively high gain to ensure relatively accurate measurement.
Many different amplifier circuit implementations have been used to sense the voltage developed across the sense resistor. The amplifier circuit is usually designed to amplify a very small sense voltage in the presence of relatively large common-mode voltages. DC precision (low input offset voltage) and high common-mode rejection ratio (CMRR) are distinguishing characteristics of the op amps used within amplifier circuits.
It is often desired to measure current in both directions so that a bidirectional amplifier circuit is used. One common bidirectional configuration is to use a pair of amplifiers (e.g., operational amplifiers) in a dual configuration in which a first amplifier senses current in one direction and a second amplifier senses current in the opposite direction. In certain applications, however, it may be desired to minimize the size and cost of circuitry and power consumption. A dual amplifier solution typically increases cost, size and power consumption, and each individual amplifier usually has a voltage offset. The dual amplifier solution should be designed to ensure that only one amplifier is on at any given time to prevent erroneous output or even circuit failure. This presents a design challenge at the low level or zero-crossing point, since the dual configuration is usually configured with a voltage gap thus reducing overall accuracy.
Single amplifier implementations are known. Such implementations are either complicated, expensive, power consuming, etc. For example, the power must usually be increased to increase bandwidth (speed of response), and complicated amplifier topology along with filtering circuitry is usually added to minimize offset voltage. It is desired to provide a simple, low power solution which minimizes offset voltage to improve accuracy.